I examined genetic exchange (transformation) between heterologous and "homologous" plasmid DNAs and the group H streptococci, strains WE4, Wicky, and Challis. The three strains were transformed to erythromycin of lincomycin resistance when treated with plasmid DNAs derived from streptococcus pyogenes strain SM 60, S. faecalis strain DS5, and Bacillus subtilis strain BD431. These strains harbor ERL1, pAM beta 1, and pBD15, respectively. The transformation order for heterologous plasmids was: ERL1 was greater than pAM beta 1 which was greater than pBD15. Plasmids isolated from transformed clones (homologous plasmids) transformed the wild-type parents at higher frequencies. An explanation for this is that the surviving plasmids are enzymatically modified.